# std::ios_base::iword

From cppreference.com
< cpp‎ | io‎ | ios base

long& iword( int index ); |  |

First, allocates or resizes the private storage (dynamic array of long or another indexable data structure) sufficiently to make index a valid index, then returns a reference to the long element of the private storage with the index index.

The reference may be invalidated by any operation on this `ios_base` object, including another call to `iword()`, but the stored values are retained, so that reading from iword(index) with the same index later will produce the same value until the next call to std::basic_ios::copyfmt(). The value can be used for any purpose. The index of the element must be obtained by a previous call to xalloc(), otherwise the behavior is undefined. New elements are initialized to ​0​.

If the function fails (possibly caused by an allocation failure) and *this is a base class subobject of a `basic_ios<>` object or subobject, calls std::basic_ios<>::setstate(badbit) which may throw std::ios_base::failure.

### Notes

Typical use of iword storage is to pass information (e.g. custom formatting flags) from user-defined I/O manipulators to user-defined `operator<<` and `operator>>` or to user-defined formatting facets imbued into standard streams.

### Parameters

index | - | index value of the element

### Return value

A reference to the element.

### Exceptions

May throw std::ios_base::failure when setting the badbit.

### Example

Run this code

    #include <iostream>
    #include <string>

    struct Foo
    {
        static int foo_xalloc;
        std::string data;

        Foo(const std::string& s) : data(s) {}
    };

    // Allocates the iword storage for use with Foo objects
    int Foo::foo_xalloc = std::ios_base::xalloc();

    // This user-defined operator<< prints the string in reverse if the iword holds 1
    std::ostream& operator<<(std::ostream& os, Foo& f)
    {
        if (os.iword(Foo::foo_xalloc) == 1)
            return os << std::string(f.data.rbegin(), f.data.rend());
        else
            return os << f.data;
    }

    // This I/O manipulator flips the number stored in iword between 0 and 1
    std::ios_base& rev(std::ios_base& os)
    {
        os.iword(Foo::foo_xalloc) = !os.iword(Foo::foo_xalloc);
        return os;
    }

    int main()
    {
        Foo f("example");
        std::cout << f << '\n' << rev << f << '\n' << rev << f << '\n';
    }

Output:

    example
    elpmaxe
    example

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 36 | C++98 | the stored value might not be retained if the reference is invalidated | the stored value is retained until the next call of `copyfmt()`
LWG 41 | C++98 | the function set badbit by itself on failure, but `ios_base` does not provide such interface | badbit is set by `basic_ios` (if *this is its base class subobject)

### See also

pword | resizes the private storage if necessary and access to the void* element at the given index (public member function)
xalloc [static] | returns a program-wide unique integer that is safe to use as index to `pword()` and `iword()` (public static member function)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/io/ios_base/iword&oldid=159114"